This invention relates to cast plate straps for lead-acid storage batteries, and more particularly, to antimony-free plate strap castings for so-called "maintenancefree", lead-acid batteries which, due to their non-antimonial or low-antimonial grid alloys, have a low self-discharge characteristic and lose little water during normal charge-discharge cycling.
The modern SLI lead-acid storage battery comprises essentially a case which is divided into a plurality of individual compartments or cells by partitions extending transverse the case. Each of these cells contains a plurality of electrochemically active plates comprising alternately stacked negative plates and positive plates separated one from the other by a microporous separator. Each plate includes an electrically conductive (e.g., metallic) skeletal support or grid for the electrochemically active plate material, which grid includes a conductive tab or lug projecting beyond (i.e., usually above) the periphery of the stack for electrically connecting each grid to like grids of the same polarity within the stack. In conventional fashion, the lugs for the positive plates are aligned each with the other and likewise for the lugs of the negative plates. Each aligned set of lugs are then electrically joined by fusion to a cast plate strap (i.e., either cast-on or pre-cast and burned on). The plate strap typically includes an intercell connector lug on one end thereof which serves to electrically join a plate strap of one polarity from one cell to a plate strap of opposite polarity in the next adjacent cell/compartment. This connection is popularly made beneath the cover for the case and most usually directly through an aperture formed in the partitions at about the plate strap level and below the electrolyte (i.e., H.sub.2 SO.sub.4) level of the battery (e.g., See Matter et al U.S. Pat. No. 3,947,290).
There is a current trend toward making such batteries "maintenance-free", in that they require no water additions during their normal useful lives. Such maintenance-free batteries have been made possible by minimizing or eliminating antimony from the grid alloy of the battery plates. By substantially removing the antimony from the grids, the hydrogen overvoltage of the cell is raised and water loss by electrolysis during overcharge is significantly reduced to the point where the battery can be sealed (i.e., except for venting) with only some excess electrolyte provided to insure adequate service life. Typically modern maintenance-free grid alloys comprise Pb-Ca-Sn alloys. Though antimony has been substantially removed from the grid alloy, some manufacturers have retained significant amounts of it (i.e., 2% or more) in the plate straps which electrically join the positive plates together and the negative plates together in each cell of the battery. In this regard, the antimony improves castability and provides strength to the plate strap so that the strap and the intercell connector made from the strap lug can resist the high forces they experience in use primarily during periods of battery vibration. The antimony from the plate strap, however, can slowly dissolve into the electrolyte and eventually deposit on the negative plate which ultimately results in an increased tendency toward electrolysis of the water, which in turn can be translated into potentially shortened useful life of the battery. By removing substantially all antimony from the battery then, not only can life be extended, but the present requirement for extra electrolyte in the sealed batteries can be reduced, which in turn translates to reduced cost and lower weight batteries.